Validate-docs script return exit 0 status even though
airshipctl command fails in variable assignment with
broken pipe. This make validate-docs zuul gate passes
without complete script execution. In order to solve
this issue added bash option 'set -o pipefail' which
ensure return exit 0 if all commands in the pipeline
exit successfully.
Also minor change in echo statement which should print
'successful site valiation' only when particular phase
evaluate not for skipped phases.
Change-Id: I24db77e8054f4c7017d533b6640516f3141675fc
98a7976b73